Nutrient balances in terraced paddy fields under traditional irrigation in Indonesia

In Indonesia, rice is not only staple food, but also a source of income, providing job for the villagers. In the past, most studies focused on lowland rice using half or fully regulated technical irrigation systems to increase rice production and improve farmers’ income. Land conversion to non agricultural purposes, competitions in water demands with other industries, and socioeconomic and biophysical problems resulted in shrinking total surface area available for cultivating rice. Hence, intensifying terraced paddy fields became very important. The current study, therefore, focuses on these. The mobility of sediments and nutrients, nutrient uptake during rice growth and the effect of rice straw addition on rice production were evaluated. Sediment was found to be mainly displaced during harrowing in the terraced paddy fields under traditional irrigation, both in the wet and dry seasons. In contrast, sediments were deposited on every terrace a week before and after fertilising. Both at harrowing and during fertilising, the amounts of incoming dissolved nutrients were higher than outgoing. It may be concluded that terraced paddy field systems are not only the way to produce rice, but also provide environmental services. It is strongly recommended to protect these areas to reduce erosion and other downstream negative effects. Application of 100 kg ha-1 season-1 of urea, TSP and KCl along with 33 % of the rice straw produced in the previous growing season always results in the highest N, P, and K concentrations in shoot and roots, nutrient uptakes and significantly increased rice yields, both in the wet and dry seasons. However, application rates of 100 kg ha-1 season-1 of urea, TSP and KCl along with 33 % of the rice straw produced in the previous growing season presents positive P balances, but negative N and K balances. This illustrates that urea and KCl application rates are not sufficient to replace N and K removed by grains and straw. Towards sustainable rice farming in terraced paddy field systems, application rates of 200-250 kg of urea, 100 kg of TSP and 200 kg of KCl ha-1 season-1 along with recycling rice straw for at least 33 % of its production are highly recommended. Scaling up this study from farm to district level shows that when a minimal rice yield of 5 t ha-1 is expected and 33 % of the rice straw production is recycled, the current recommended fertiliser rates remain economically and environmentally valid. However, at an expected rice yield of 5 t ha-1 and when no rice straw is recycled, application rates of 280 kg of urea, 100 kg of TSP and 300 kg of KCl ha-1 season-1 are needed. Conventional farmer practices with a urea application of 50 kg ha-1 season-1 always show the lowest nutrient concentration in the rice, the lowest rice yield and negative nutrient balances. Hence, these practices may not longer be recommended. Realising that intensification of livestock production also needs rice straw, the use of 67 % of the rice straw production as cattle feed and returning 33 % to the field may be a realistic recommendation